Milotic

The UnderUsed tier is filled to the brim with bulky Water-types, and all of them fill some sort of niche the others lack; Blastoise has the valuable Rapid Spin, Slowbro and Slowking boast Fighting-type resistances and nearly endless recovery with Regenerator, Suicune has more overall bulk than all its competitors, and Empoleon and Swampert have unique typings and access to Stealth Rock. It's getting harder to see the niche Milotic holds, but it alone has access to instant recovery and spectacular Special Defense with a pure Water typing. This means that it fills a role similar to Slowking's, but it trades Regenerator for a neutrality to Heracross's Megahorn and Yanmega's Bug Buzz. Also, unlike many bulky Water-types, it has a decent base Speed. Despite having a rather weak Defense, Milotic can still be used on any team needing a check to several of the tier's top threats, but if you're looking for a hard counter, you should look elsewhere.